This invention relates to a smoke detector which is effective when used as a fire detector.
A number of smoke detectors of ionization type or optical type have been manufactured. In the ionization smoke detector, a pair of electrodes across which a voltage is applied are provided in a detection chamber, and the internal atmosphere of the detection chamber is ionized by radio-active rays, so that smoke is detected from the ionization current. On the other hand, in the optical smoke detector, a light beam from a light source is introduced into a detection chamber, and a light receiving element detects the light beam which is subjected to scattering or attenuation by smoke, to thereby sense the smoke.
The smoke detectors described above frequently operate erroneously when an insect enters into the detection chamber. This will be described in more detail. In the ionization smoke detector, a pair of electrodes are provided in a reference chamber into which smoke does not substantially flow, and the internal atmosphere of the reference chamber is ionized by radio-active rays. Furthermore, a pair of electrodes are provided in a detection chamber into which smoke flows, and the internal atmosphere of the chamber is also ionized by radio-active rays. These two chambers are connected in series to each other, and the variation of the potential at the connection point is detected to sense the presence of smoke. When smoke enters into the detection chamber, the ionization current is reduced. This reduction is detected as potential variation. In general, the distance between the electrodes in the reference chamber is very small. Therefore, even a small insect can short-circuit the electrodes to abruptly change the potential, i.e., to cause the smoke detector to operate erroneously. Furthermore, when an insect sticks to the radio-active source for ionizing the internal atmosphere of the detection chamber to obstruct the ionization, the smoke detector is caused to operate erroneously.
An insect is also the largest cause for the error of the optical smoke detector, because the insect largely reflects or intercepts light.
In order to prevent the erroneous operation due to the entrance of an insect, a smoke detector has been proposed in the art in which a differentiation circuit is provided between a detection section and a signal processing section, so that a signal due to smoke is distinguished from a signal due to insect, to thereby control the signal processing section. Another smoke detector employs a method in which the signal from the detection section is supplied directly to the central monitoring section, so that it is discriminated according to the rise characteristic thereof. However, these conventional smoke detectors still suffer from problems such that the circuitry is intricate and the installation is expensive.